1. Field of the Invention
The present invention relates to a semiconductor memory with a resistance change element as a memory cell.
2. Description of the Related Art
In recent years, performed energetically is a development race of a universal memory device which, despite being nonvolatile memory cell, has properties of high speed, high integration, low power consumption and high reliability. Among them, a semiconductor memory with a resistance change element as a memory cell, for instance, a magnetic random access memory with a magnetoresistive element as a memory cell is one of major candidates to achieve practical application (Refer to, for instance, U.S. Pat. No. 5,695,864, Jpn. Pat. Appln. KOKAI Publication No. 2004-348934, and “16 Mb MRAM Featuring Bootstrap Write Driver” 2004 Symposium on VLSI Circuits Digest of Technical Paper, pp. 454-457).
The magnetic random access memory enables the storage of binary data by utilization of the magnetoresistive effect. At present, as the magnetoresistive effect, it is general to utilize a tunneling magnetoresistive (TMR) effect. The TMR effect is realized by a magnetic tunnel junction (MTJ) element composed of a tunnel barrier layer and two ferromagnetic layers sandwiching the tunnel barrier layer therebetween.
Then, in the case where the magnetization directions of the two ferromagnetic layers of the MTJ element are in the same direction (parallel state), tunnel probability of the tunnel barrier layer becomes maximum, while the resistance of the MTJ element becomes minimum. This state is regarded as, for instance, “1”. Further, in the case where the magnetization directions of the two ferromagnetic layers of the MTJ element are in opposite directions (anti-parallel state), tunnel probability of the tunnel barrier layer becomes minimum, while the resistance of the MTJ element becomes maximum. This state is regarded as, for instance, “0”.
Here, as for data writing (magnetization reversal) to the MTJ element, a magnetic field writing system utilizing the magnetic field generated by a current flowing through a write line, and a spin momentum transfer writing system utilizing a spin torque due to a flowing of spin-polarized electrons through the MTJ element are known.
The magnetic field writing system has characteristics that switching magnetic field necessary for the magnetization reversal becomes large in proportion to miniaturization of the MTJ element. For this reason, there is a problem that, in the magnetic field writing system, when conversion efficiency to a magnetic field from a current is constant, the value of the write current becomes large due to scaling down of the MTJ element.
On the contrary, in the spin momentum transfer writing system, magnetization reversal is performed utilizing the spin torque due to the spin-polarized electrons. Then, the magnetization reversal is generated when current density of the spin injection current flowing through the MTJ element exceeds a constant value. That is, the spin momentum transfer writing system, provided that the current density is constant, has characteristics that the value of the spin injection current becomes small in proportion to the square of the length reduction ratio of the MTJ element.
Therefore, the spin momentum transfer writing system has becomes one of the major techniques for achieving practical application of the magnetic random access memory.
However, the biggest problem of the spin momentum transfer writing system is decrease of current density of the spin injection current necessary for the magnetization reversal.
For instance, in a so-called 1Tr-1MTJ type in which one memory cell is composed of one MOSFET and one MTJ element, there is a problem that sufficient current density is not obtained in the course of advance of miniaturization of the memory cell.
As one of the techniques for solving the problem, there is a technique so-called 2Tr-1MTJ, in which one memory cell is composed of two MOSFETs and one MTJ element. However, in this case, one transistor increases per memory cell, and naturally, cell size becomes large corresponding thereto, which is a disadvantage for high integration.